Safety circuit for cathode ray tubes



p 1941- E. G. GAGE SAFETY CIRCUIT FOR CATHODE RAY TUBES File d Sept. 1,'1959 INVENTOR. D 5011641?!) 646E BY 41- m AZZLM,

ATTORNEY.

Patented Sept. 2, 194i NETED STATES PATENT oFFlcE" SAFETY CIRCUIT FORCATHODE RAY TUBES Edward G. Gage, Brooklyn, N. Y., asslgnor, by directand mesne assignments, 01 two-thirds to Leon Ottinger, New York, N. Y.

Application September 1, 1939, Serial No. 292,993

11 Claims.

The invention relates to a novel system of sup plying a high voltage tothe anodes of cathoderay tubes and other thermionic devices such asradio tubes; and more especially in a manner such that any exposed partof the wiring of the circuit may be contacted Without personal injury.

An object of the invention is to allow the handling without injury ofthe circuits of a cathode ray or radio tube operating at dangerousvoltages.

Another object is to provide a transformer so constructed that,regardless of the voltage in the secondary, it may be contacted at anyexternal part without injury.

Still another object is to provide a filter condenser so constructedthat it is impossible to obtain a dangerous shock either from a residualcharge or from a charge in the condenser due to the impressed voltage.

Other and ancillary objects will hereinafter appear.

In the operation of cathode ray tubes, it is well known that therequired voltages, especially those employed in television, are of sucha high value as to be extremely dangerous and even deadly when the livecircuits are contacted.

Voltages of this nature may range from 400 for the smaller cathode raytubes to approximately 5,000 for the larger sizes, such as are used fortelevision.

The ordinary methods of protection to the operator, as practicedheretofore, consist in providing interlocking switches so arranged thatthe cover of a cabinet containing the apparatus must be removed to getat the wiring; and when this is done, the primary current energizing thesame is interrupted.

The objection to this method is the fact that as the primary circuit isbroken, it is not possible to properly investigate the circuits fortrouble with the current on. The usual procedure, therefore, is toconnect a jumper across the switches temporarily, thus defeating thepurpose of the protective devices.

-Grounding of the high potential anode is another method employed toprotect the operator but the objection to this method is the fact thatif the ground wire becomes disconnected, or one of the condensersseparating the live side of the ment of a cathode ray tube to begrounded in the ordinary manner, and the. high potential anode to beleft clear of the ground connection, and yet prevent dangerous shock.This is accomplished in the manner hereinafter set forth, bearing inmind that a cathode ray tube anode draws an extremely small current, afew mils only. The drop across the line supplying this voltage isordinarily extremely small because of the extremely high resistance ofthe anode circuit compared to the resistance of. the line circuit.

The conventional transformers supplying voltage to the line, throughrectifiers, are wound with copper wire of small size. Even the smallestpractical sizes, however, are capable of delivering a dangerous current,if sufficiently low' resistance is placed across the secondary, such asthe human body.

In addition, the ordinary condensers as used in filter circuits arecapable of retaining a very heavy residual charge after the impressedvoltage has been removed. While the impressed voltage is supplied, amuch heavier current may be drawn from the filter condenser momentarilythan from the charging circuit without the condenser. This is one of thereasons why the ordinary filter circuit is dangerous to contact.

' To overcome these objections, and to make it impossible to draw adangerous current from any part of the cathode ray and like high tensioncircuits, the novel arrangement contemplates winding the secondary ofthe transformer supplying the high voltages with fine resistance wireinstead of the usual low resistance copper wire. Or, the usual copperwinding may be utilized and additional series resistance included in theoutput connections to the terminals, the transformer or transformer andadditional resistance being encased to prevent contact with portions oflow resistance at high voltages.

This permits a flow of suflicient current to operate the beam of thetube, which current is of a harmless value, or, it may operate the platecircuit of a radio tube; but the introduced high resistance absolutelyprevents a larger current i from being drawn through the body ifcontacted,

at the moment of contact by the body, the apparatus voltage "momentarilydrops from 5,000 volts, as an example, to approximately 100. Uponremoval of contact, the voltage again rises to 5,000.

The resistance of the human body varies greatif its resistance should beless than 5,000 ohms or' even of negligible resistance.

A better understanding of the safety provision may be had by referringto the drawing, in which:

Fig. 1 represents a schematic drawing of the novel arrangement asapplied to a cathode ray tube used, as an example, for television.

Fig. 2 shows a cross-section of a transformer protected in the novelmanner; and Fig. 3 is a similar view of a protected filter condenser.

Fig. 4 shows a schematic drawing of the novel arrangement as applied toa radio tube.

' Referring to the drawing, more especially Fig. 1, i designates thebody of a cathode ray tube com prising the filament ii, the controlanode l2, the first anode iii, the second anode it, the verticaldeflecting plates i5 and i6, and horizontal defleeting plates ii andiii. A filament transformer supplies current to the filament ii througha filament control rheostat El; and for the control electrode there isprovided a bias battery 22.

, Reference character 23 designates the first anode transformer, 20 thefirst anode voltage primary control, and 25 the first anode rectifier,while the first anode rectifier filament transformer is designated bythe reference character 26 and the first anode safety condenser by thereference character if. Across this condenser is connected a shuntresistance 20, and two protective resistances 20 and 30 are connected inseries with the condenser terminals in accordance with the invention. Asecond anode transformer 3i having the primary control 02 is connectedto the second anode rectifier 33, while.the second anode rectifierfilament transformer 30 is connected to the filament thereof. A secondanode safety condenser is designated 'by the reference character and thesame is provided witha shunt resistance 06 and the series protectiveresistances 31 and 36. A ground return connection 39 is also providedtherefor and a ground return connection 00 for condenser 2?. Powersupply for all of the transformers is obtained from the mains 4i; andall of the low resistance, high voltage portions of the apparatus are tobe housed in a suitable casing to prevent contact therewith, only thehigh resistance terminals remaining exposed.

Thus, as indicated in Fig. 2, 05 indicates the grounded casing of thesafety transformer with core 46, primary winding 41, and secondarywinding 48 housed therein, together with internal protective resistances49 and 50 which are connected to the respective external binding posts5! and 52.

In Fig. 3, represents the grounded case of the safety filter condenserhaving the electrodes or coatings 50 and 51. connected to the respectiveterminals 58 and 59. A shunt resistance 00 is connected across theterminal leads within the casing, and there is included in therespective leads the series safety resistances 6i and 62.

In Fig. 4 there is indicated an ordinary threeelement radio tube 65,which may also be a multielement tube, the same comprising the plate 66,the grid 61,, cathode 6B, and the heater 69. A

high voltage safety transformer 10 therefor, with primary II andsecondary I2, is arranged with the internalsafety resistances 13 and M,while a winding 15 is provided for heating the filament assesses E0 of arectifier ii. The filter condenser i0 is made safe by the seriesintroduction of the protective resistances i0 and 80, and a shuntresistance at is bridged across the same. The usual choke coil orimpedance is indicated at 02.

In the practical application of my invention to medium-sized cathode raytubes, it is pre ferred to employ resistances of the order of 20,000ohms in the secondary of the safety transformer, or sufficient internalresistance may be added in series to that value to allow a maximum of,10 mils to pass through the secondary when short circuited.

It is apparent that the higher the voltage required to operate the tube,the higher must be the resistance of the secondary. No. 36 Nichromeresistance wire is a practicalsize wire with which to wind the secondaryto obtain the above results. If copper wire is used for the secondary,then the internal resistances 49 and 50, Fig 2, should be increaseduntil the total rmistance is sufiicient to pass but 10 mils onshort-circuit.

'I'heinternal resistances 0i and 62, Fig. 3, of the safety filtercondenser may be of the order of 10,000 ohms each, while the shuntresistance 00, Fig. 3, may be of the order of l megohm. This shuntresistance allows a residual discharge to leak slowly from thecondenser, thus preventing shock from a later contact.

The resistances M and 02 prevent a heavy surge of current from passingfrom the condenser through contact of the body with its terminals.

In the application of the invention to ordinary radio tuibes, where theplate voltage is comparatively low, of the order of 500 volts only, theresistances of the secondary, of the transformer may be of the order of2,500 ohms each, or of sufficiently high resistance to permit the tubeto operate with sufficient plate current, and yet prevent a dangerous(over 50 mils) current. to flow. The same circuit may also be applied tovacuum tube transmitters having high imped-,

ance tubes.

While I have shown specific adaptations of my invention, it is apparentthat many variations of the safety circuits described herewith may bemade without departing from the scope of the appended claims. I claim:L-In the art of safeguarding the human body with respect to exposedparts of a dangerously high. potential electrical circuit connected witha high potential source to render such parts harmless upon bodilycontact with the same: an electrical circuit including atransformersecondary winding afl'ording a. dangerously high potential,a. cathode raytube connected therewith, a high potential filtercondenser in the connection, and 9. rectifier interposed between thetransformer and the condenser, a casing enclosing the said transformerand a casing enclosing the said condenser, resistors located within therespective casings and of a value sumciently low to permit operat on ofthe said tube but of a value sufficiently high to effect a drop, whenbodily contact is made with the said circuit externally of therespective casings, of voltage in the circuit to a degree sufficient tolimit the passage of current through said circuit and body to a harmlessvalue.

2. In the art of safeguarding the human body with respect to exposedparts of a dangerously high potential electrical circuit connected witha high potential source to render such parts harmless upon bodilycontact with the same: an electrical circuit including. a transformersecondary winding affording a dangerously high potential, a cathode raytube connected therewith, a high potential filter condenser in theconnection, and a rectifier interposed between the transformer and thecondenser, a casing enclosing the said transformer and a casingenclosing the said condenser, resistors located within the transformercasing in series in the respective output leads from its secondary andresistors located within the casing for the condenser in series, in therespective output leads from its electrodes, and a further resistorconnected across the condenser output leads within the casing of a valuesufflciently high to prevent an appreciable drop of voltage across thecondenser leads and sufficiently low to prevent accumulation of 'aresidual charge, said series connected resistors being respectively of avalue suflic ently low to permit operation of the said tube but of avalue sufdciently high to eflect a drop, when bodily contact is madewith the said circuit externally of the respective casings, of voltagein the circuit to a degree sufiicient to limit the passage of currentthrough said circuit and body to a harmless value.

3. In combination, a transformer having a low voltage input winding, anenclosing casing for said transformer, together with terminals carriedby said casing externally thereof, an intermediate output high voltagecircuit portion of a resistance value sufliciently low to permitoperation of electrical devices from the terminals but of sufficientlyhigh value to prevent passage of current of dangerous value, a highresistance electrical device connected with the respective terminals,and a filter condenser, together with a rectifier therefor, saidcondenser comprising a casing for said condenser, terminals carried bysaid casing externally thereof and connected with the terminals of saidtransformer through the said rectifier, and resistance inserted betweenthe condenser electrodes and said external condenser terminals withinsaid casing, and being of sufiiciently low value to allow the operationof the electrical device from said condenser terminals, but ofsufficiently high value to prevent the passage of current of a dangerousvalue.

4. The combination with an e ectrical condenser and an enclosing casingtherefor having denser and an enclosing casing therefor having externalterminals connected with the condenser electrodes, and a resistorconnected across the condenser terminals within the casing of a valuesufliclently high to prevent an appreciable drop across the condenserleads and sufiiciently low to prevent accumulation of a residual charge;of resistance located within the casing and connected intermediate theelectrodes and terminals and of a value suificiently low to afford therequired potential at the said terminals but of a value sufllcientlyhigh to effect a drop of voltage, when bodily contact is made with thecircuit at or beyond the said terminals, to a degree sumcient to limitthe passage of current through the body to a harmless value. v

7. An electrical circuit having a source of danger'ously high potential,a high-resistance device operating at a dangerously high potential,conductors between the source of high potential and a the said deviceaffording a permanently continuous connection therebetween, andresistance permanently included in said conductors so close to saidsource of high potential that all exposed parts of the circuit liebetween the resistance and said device, said resistance being of suchhigh magnitude that upon establishment of a closed circuit through saidexposed parts and the human body the current in such closed circuit isreduced by said resistance to a safe value.

8. An electrical circuit having a transformer affording a dangerouslyhigh potential, a cathode ray tube operating at a dangerously highpotential, conductors between the former and the latter affording apermanently continuous connection therebetween, and resistancepermanently included in said conductors so close to said transformerthat all exposed parts of the circuit lie between the resistance andsaid cathode ray tube, said resistance being of such high magnitude thatupon establishment of a closed circuit through said exposed parts andthe human body the current in such closed circuit is reduced by saidresistance to a safe value.

9. An electrical circuit having a source of dangerously high potential,a device having a resistance of the order of magnitude approximatingthat of the human body and operating atsuch dangerously high potential,conductors beexternal terminals connected with the condenser electrodes;of resistance located within the casan enclosing casing for thecondenser having ex ternal terminals connected with the condenserelectrodes; of res stance located within the cas ing and connectedintermediate the electrodes and terminals and of a value suiiicientlylow to afford the required potential at the said terminals but of avalue sufilciently high to effect a drop of voltage, when bodily contactis made with the circuit at or beyond the said terminals to a. degreesufflcient to limit the passage of current through the body to aharmless value.

6. The combination with an electrical contween the source of highpotential and said device affording a. permanently continuous connectiontherebetween, and resistance permanent-, ly included in said conductorsand the said circuit having all of its exposed parts lying between saidresistance and said device, the resistance of the circuit between saidsource and any of said parts being of a magnitude sufficiently low toenable said device to be operated at the required voltage but so high'in comparison to the resistance of the human body that uponestablishment of a closed circuit through said exposed parts and thehuman body the current in ieest one conductor from each oi! saidtransformer secondary windings so close to a said winding that allexposed parts or the circuit lie between the resistance and said device,said resistance being of such high magnitude that uponv ,esteblint of aclosed circuit through said vo, high potcntiel source to render suchparts harmless upon bodily contact with the same: an electrical circuitincluding a source of dangerously high potential, a high resistancedevice having a permanently continuous connection therewith, resistancepermanently connected with the parts oi. highest voltage, saidresistance being of such high magnitude that upon establishment of aclosed circuit through said exposed parts and the human body the currentin such closed circuit is reduced by said resistance to e. neie value,and means to encase the said source and said resistance.

EDWARD G. GAGE.

